my thermostat is stuck, how do i locate it

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My radiator seems to be damaged, the car overheated and when I opened the hood it was smoking and the fluid reserve was boiling and leaking. I tried letting it cool off then putting more fluid in the resivoir but it just goes from half full to empty. Can't findd the thermostat. It was fine, 156k miles, yesterday it just went all crazy. Overheats in about 15 min, sometimes it goes back down, but then eventually overheats again. Sometimes radiator light comes on when temp gauge seems fine. Always flashes when temp is high. I'M LOST, READY TO DRIVE IT IN THE RIVER!